This invention relates to a pyrolytic boron nitride article having laminated layers of anisotropic boron nitride in which a plurality of selected layers are doped with a minimum concentration of dopant sufficient to preferentially induce peeling in one or more of the selected layers and to a method for manufacturing a pyrolytic boron nitride article consisting of laminated layers of anisotropic boron nitride in which a dopant is incorporated into a plurality of selected layers for preferentially inducing peeling of one or more of the selected layers relative to a given surface of the crucible.
Pyrolytic boron nitride is formed by chemical vapor deposition using a process described in U.S. Pat. No. 3,182,006, the disclosure of which is herein incorporated by reference. The process involves introducing vapors of ammonia and a gaseous boron halide such as boron trichloride (BCl3) in a suitable ratio into a heated furnace reactor causing boron nitride to be deposited on the surface of an appropriate substrate such as graphite. The boron nitride is deposited in layers and may be separated from the substrate to form a free standing structure of pyrolytic boron nitride. Pyrolytic boron nitride is an anisotropic material having an hexagonal crystal lattice with properties perpendicular to the basal plane, known as the xe2x80x9cc-planexe2x80x9d, which are significantly different from its properties parallel to the plane, known as the xe2x80x9ca-planexe2x80x9d. Because of the high degree of anisotropy the mechanical strength of the free standing pyrolytic boron nitride (hereafter xe2x80x9cPBNxe2x80x9d) structure is weak in the perpendicular direction which is the direction of PBN layer growth.
Crucibles of PBN are used commercially to melt compounds at elevated temperatures. For example, in the production of semiconductors, crucibles of PBN are used to grow GaAs crystals. A PBN crucible is, however, subject to fracture from a build up of stress. For example when using the Liquid Encapsulated Czochralski method to produce single crystals of GaAs, fracture of the PBN crucible can occur after the crystal is grown and a boric oxide glass B2O3, which is used to cover part of the molten mass, freezes in the crucible. On freezing the boric oxide (B2O3) glass shrinks more than the crucible. The resultant shrinkage mismatch generates stresses within the PBN crucible that can cause it to fracture. Similarly boric oxide is often used as an encapsulant for GaAs in the vertical gradient freeze (VGF) method. The bond between the boric oxide and the PBN surface and the shrinkage mismatch of the boric oxide and PBN on cooling causes a radial tensile stress (perpendicular to the layers) and a compressive stress (parallel to the layers) in the PBN. This causes large stresses to develop in the PBN which, in turn, can cause delamination i.e. peeling to occur or fracture. Ideally, the PBN should peel away in thin layers of controlled thickness, providing an improved and predictable service life which would also eliminate the risk of catastrophic failure. In the past there was no way to predict to the number of layers that would peel off. For a crucible to have a long service life, it is necessary to control peeling and to preferably limit peeling to a single selected PBN layer of controlled thickness for each crystal growth run. Moreover, the single selected layer should peel uniformly from the body of the crucible when the boric oxide glass is withdrawn.
The present invention is directed to a PBN article such as a crucible of laminated layers containing anisotropic boron nitride and to a method for forming a PBN crucible having laminated layers of anisotropic boron nitride in which a plurality of selected layers contain a dopant in a predetermined minimum concentration sufficient to induce peeling of one or more selected layers in the crucible. In accordance with the present invention, the selected layer or layers contain dopant(s) of sufficient minimum average concentration to induce peeling and should be spaced a predetermined distance apart from one another. Preferably only one layer of PBN will peel off the crucible during each crystal growth operation. The dopant causes peeling to propagate along the layer plane such that peeling of the layers is uniform. The preferred spacing between the selected layers should be between about 0.1 micron and 100 microns for a crucible of given thickness to permit the number of crystal growth runs to be estimated in advance. Each of the selected layers within the PBN crucible should be doped with an elemental dopant preferably selected from the group consisting of carbon and/or oxygen, alkali metals, alkaline earth metal(s), transition metal(s), and rare earth metal(s) or selected from group 1-6 of the periodic table and/or any combination thereof with at least one of the dopants being present at a minimum average concentration of 2 atomic % when measured at a depth in a range from about 1000 to 2000 angstroms from the interfacial fracture surface (the surface of the peeled layer).
Broadly, the pyrolytic boron nitride article of the present invention comprises layers of pyrolytic boron nitride in which a plurality of selected layers contain a dopant in a concentration sufficient to induce peeling of one or more selected layers with the average minimum concentration of dopant in each selected layer being above at least 2 atomic % at a depth in a range from about 1000 to 2000 angstroms measured from the surface of the peeled layer and with each of the selected layers being spaced a predetermined distance apart of between about 0.1 micron to 100 microns. Preferably only one of the selected layers will peel off from the crucible for each use of the crucible to grow crystals in the production of semiconductors.
A pyrolytic boron nitride crucible is formed in accordance with the method of the present invention comprising the steps of introducing vapors of ammonia and a gaseous boron halide in a suitable ratio into a heated furnace reactor to cause boron nitride to be deposited in layers on a substrate, injecting at least one gaseous contaminant into the furnace at controlled periodic interval(s) such that at least two selected layers of boron nitride are doped with said gaseous contaminant(s) at a minimum concentration level of above 2 atomic % at a depth ranging from about 1000 to 2000 angstroms in each selected layer, controlling the interval of injection to space the selected layers between about 1 micron and 100 microns apart and separating the crucible from the substrate.